1. Field of the Invention
The present invention relates to broadband signal space switching apparatus and is particularly concerned with switching elements which are controlled by decoder-controlled, crosspoint-associated memory cells.
2. Description of the Prior Art
Recent developments in telecommunications technology have led to service-integrated communications transmission and switching systems for narrowband and broadband communications services which provide light waveguides as the transmission media in the area of the subscriber lines by way of which both the narrow band communications services such as, in particular, 64 kbit/s digital telephony, as well as broadband communications services such as, in particular, 140 Mbit/s picture telephony are conducted, whereby, however, narrowband signal switching devices and broadband signal switching devices (preferably having shared control devices) are provided in juxtaposition in the switching stations (German Pat. No. 24 21 002).
It is known in the context of a broadband signal time-division multiplex switching device whose crosspoints are respectively used in time-division multiplex for a plurality of connections to connect two respective lines with the assistance of a gate which is switched on and off by a crosspoint-associated memory cell constructed as a bistable D flip-flop. The crosspoint-associated memory cell has a clock input which is supplied with a corresponding clock signal and is driven in only one coordinate direction, namely at its D input (Pfannschmidt, "Arbeitsgeschwindigkeitsgrenzen von Koppelnetzwerken fur Breitband-Digitalsignale", Dissertation, Braunschweig 1978, FIG. 6.7). In view of a time-division multiplex factor of about 4-8 which is attainable given a bit rate of 140 Mbit/s and in view of complex circuit technology thereby required, however, pure space switching devices are presently preferred for switching broadband signals, the connections established via the individual crosspoints being only spacial separated from one another.
A pure broadband signal space switching arrangement can be constructed as a crosspoint matrix in whose crosspoints the switching elements are respectively controlled by a crosspoint-associated holding memory cell that is merely decoder controlled (Pfannschmidt, op. ct., FIG. 6.4). The switching elements can be respectively constructed as complementary metal-oxide-semiconductor (CMOS) transfer gates (CMOS transmission gates) (ISS'84 Conference Papers 23C1, FIG. 9). The use of a simple CMOS transfer gate as a switching element, however, involves a load of its input line by the capacitance of its output line in the conductive condition of the CMOS transfer gate, and this condition can cause signal delays. As a result of what are referred to as under-threshold effects (for example, in the form of stray currents or under-threshold currents) along the MOS transistors, however, cross-talk phenomena can occur at switching elements that are actually located in inhibited condition.
A similar case also is true of a broadband space switching device comprising crosspoints having switching elements respectively controllable by a crosspoint-associated memory cell, the switching elements being respectively constructed with a CMOS inverter circuit comprising MOS transistors of the enhancement type which has its input side connected to the appertaining signal input line and its output side connected to the appertaining signal output line, a p-channel depletion transistor having its control electrode connected to the output of a memory cell being connected between the p-channel enhancement transistor and the appertaining feed voltage source and an n-channel depletion transistor having its control electrode connected to the complementary output of the memory cell being inserted between the n-channel enhancement transistor thereof and the appertaining feed voltage source (ISS'84 Conference Papers 31C3, FIG. 14). As a consequence of a constant, at least weak conductivity of the two depletion transistors, such an execution of the switching elements leads to static dissipated power of the crosspoint, even in its inhibit condition, and also leads to the fact that a switching element which is in the conductive condition, is also loaded by switching elements which were in the inhibited condition and which are nonetheless weakly conductive. Furthermore, no full signal swings between the two values of feed potential are possible.